The present invention relates to superabsorbent polymers and to a process for preparing them. More particularly, the invention relates to superabsorbent polymers having increased absorbency in the presence of dissolved salts. In addition, a new method for forming superabsorbents in a concentrated solution is disclosed.
Superabsorbents are generally composed of high molecular weight hydrophilic polymers that have been crosslinked to provide water-insoluble but water swellable materials. Depending on the type of polymer and the type and extent of crosslinking, these polymers may be capable of absorbing many times their own weight in fluid. Although a great variety of superabsorbents based on a wide range of chemistries have been described, there are basically just two methods for producing superabsorbents. One method involves polymerization of low molecular weight precursors to provide superabsorbents directly; the second method involves the crosslinking of preformed polymers. In forming superabsorbents by either method, the control of the amount of crosslinking (crosslink density) is very important with regard to the performance of the superabsorbent.
One of the problems associated with crosslinking of preformed polymers is that typically the polymer is of high molecular weight and when these materials are dissolved in water, they form very viscous solutions even at low concentrations. In order to thoroughly mix the crosslinker with the polymer, the solids in solution must be fairly dilute, on the order of 0.5% or less. Such extensive dilution requires large tankage capacity and the expenditure of much money, energy and time to rid the final polymer product of the water necessary to make it. Therefore, there is a need to develop a process wherein the polymer materials can be mixed at higher solids concentrations, thereby reducing the amount of water needed in the process and achieving savings in energy, size of equipment, effluent control and manufacturing time.
The present invention provides a method to form crosslinked superabsorbents at relatively high solids concentrations of polymer wherein low molecular weight, essentially non-ionic, hydrophilic molecules can be incorporated and immobilized. Incorporation of hydrophilic molecules, such as starches and saccharides in the composition, reduces both the cost of the superabsorbent as well as the sensitivity of the superabsorbent composition to dissolved salts in the aqueous medium. It also provides increased flexibility to the final composition. The superabsorbents thus produced may be used in thin film, powder, in a bulk form (thick film or plaque), or as a coating or binding for paper or nonwovens.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.